Asphalt Emulsions 101

By Dwight Walker

Asphalt Emulsions 101Asphalt is one of the more commonly used construction materials. It is widely used in pavements to bind and waterproof aggregate mixtures and to seal surfaces. Asphalt is used in hot mixes and now warm mixes. And there is another major category of asphalt applications — those including the use of asphalt emulsions.

The Asphalt Institute?s manual, “MS-26 -The Asphalt Binder Handbook,” defines emulsification as “a process through the use of certain mechanical and chemical processes, that allows two or more materials to combine that will not mix under normal conditions.”

Emulsions are very common materials; ice cream is an example. An asphalt emulsion consists of three basic components, asphalt, water and an emulsifying agent.

An asphalt emulsion is a stable dispersion of asphalt cement droplets in water, which can be pumped, stored and mixed with aggregates. An emulsion should “break” quickly upon contact with aggregate when mixed or after being sprayed. Breaking is the separation of the water from the asphalt. After breaking and curing, the asphalt residue has the adhesion, durability and water-resistance properties of the original base asphalt.

Classifications and additives

The American Society for Testing and Materials (ASTM) and the American Association of State Highway and Transportation Officials (AASHTO) have standardized how emulsions are identified. (Most highway agencies have adopted these standards, but there are some local and/or proprietary designations.) The first designation is based on electrical charge. Cationic (positively charged) emulsions are identified with a “C.” If the C is not included, the emulsion is normally anionic, negatively charged. There are a few nonionic emulsions, but they are not widely used.

After the charge designation in an emulsion classification, there is information about the “setting” rate. Setting is closely related to how soon an emulsion breaks. The standardized designations are RS for rapid set, MS (medium set), SS (slow set) and QS (quick set). RS emulsions break rapidly and are typically used for spray applications, such as chip seals and sand seals. MS emulsions are designed to be mixed with graded aggregates. MS emulsions are used in making cold mixes and in cold recycling applications. SS emulsions are designed for maximum mixing time and workability. They are the most stable emulsions and are used for tack or bond coats, fog sealing as well as in slurry seals and some asphalt surface courses. QS emulsions are designed for use in micro-surfacing and slurry seals when a quick curing time is needed for opening to traffic.

The addition of “P” in the emulsion classification indicates polymer has been added to the emulsion. “L” means a latex polymer has been used. Polymers are added to enhance strength, adhesion and durability. Numbers in an emulsion’s classification refer to the viscosity of the emulsion. A “2” (for example, RS-2) is more viscous than a “1”(RS-1). An “h” indicates harder base asphalt was used (SS-1h). “HF” or high float emulsions use a gel structure to allow a thicker asphalt film.

Storage, handling and testing

Emulsions are made in specialized plants, usually including a colloid mill. The asphalt is sheared into tiny droplets (1 to 10 microns, smaller than a human hair) and blended with the other components. Emulsions have particular storage and handling requirements. Some important handling guidelines include:

  • Store most emulsions between 50 and 185¿F. Some grades have more specific storage limits. Storage tanks should be insulated to reduce heating costs and to guard against freezing. Do not allow emulsions to boil. 
  • Avoid excessive agitation or pumping. Use gentle, slow-speed circulation techniques. 
  • Avoid mixing different classes, types and grades of emulsions in storage tanks, haul transports or distributors. 
  • Do not dilute rapid sets with water. Medium sets and slow sets can be diluted by slowly adding water to the emulsion. 
  • As with any material, accurate test results are dependent on proper sampling and handling techniques. Obtaining representative samples of emulsions is critical. Experienced, knowledgeable personnel should take samples. Contamination and/or premature breaking are specific concerns for emulsion samples. 

Some emulsion tests are intended to measure composition, consistency, and stability; other tests on the asphalt residue predict performance in service. Standardized emulsion tests are documented in ASTM D 244 and AASHTO T 59.

More detailed information on care and handling of emulsions is available in manuals available from the Asphalt Institute. Specific questions about storing, handling and sampling of emulsions should be referred to the supplier.


Emulsions are extremely versatile materials. They are used for preventive and corrective maintenance activities on both asphalt and concrete pavements. They are used in stabilizing and reclaiming base courses, for constructing structural layers within a pavement and for various surface treatments. Emulsions can be used in recycling old, deteriorated pavements.

Some of the specific uses of emulsions include:
Tack (also known as bond) coats

  • Fog seals
  • Chip seals and other surface treatments
  • Slurry seals and micro surfacing
  • Recycling (hot-in-place, cold-in-place, etc.)
  • Cold mixes.

Information and resources

For those interested in learning more about asphalt emulsions and their use, numerous resources are available. The Asphalt Institute and the Asphalt Emulsion Manufacturers Association (AEMA) have jointly developed “MS-19 – The Basic Asphalt Emulsion Manual”, now in the fourth edition. Other sources include the International Slurry Surfacing Association (ISSA), the Asphalt Recycling and Reclaiming Association (ARRA) and the Foundation for Pavement Preservation (FP2.)

Asphalt emulsions offer the environmental advantages of low emissions and reduced energy usage, as well as having safety benefits. The wide range of available formulations and the numerous service applications make emulsions extremely versatile. These characteristics make asphalt emulsions a good fit for today’s economic and environmental concerns.

Dwight Walker is a contributing editor for Asphalt magazine and is a consulting engineer specializing in asphalt materials and construction.